This invention relates generally to fabricating hollow airfoils and more particularly concerns a method of producing lightweight, high-strength hollow airfoils using diffusion bonding and superplastic forming techniques. This method is particularly useful in making hollow titanium aircraft engine blades for integrally-bladed rotors.
Superplastic forming is a technique that relies on the capability of certain materials, such as titanium alloys, to develop unusually high tensile elongation with a minimal tendency towards necking when submitted to coordinated time-temperature-strain conditions within a limited range. Superplastic forming is useful in producing a wide variety of strong, lightweight articles.
Many of the same materials used in superplastic forming are also susceptible to diffusion bonding. Diffusion bonding is a process which forms a metallurgical bond between similar parts which are pressed together at elevated temperature and pressure for a specific length of time. Bonding is believed to occur by the movement of atoms across adjacent faces of the parts. Diffusion bonding provides substantial joint strength with little geometrical distortion and without significantly changing the physical or metallurgical properties of the bonded material.
It has long been desirable to fabricate various aircraft components, such as door panels and wing flaps, as hollow bodies. The benefits of such include a substantial reduction in weight which provides improved fuel efficiency and increased thrust-to-wight ratio. Despite the increasing popularity in applying diffusion bonding and superplastic forming (DB/SPF) techniques to the manufacture of aircraft components, there are many critical problems to overcome in successfully forming a hollow airfoil. Parts formed using DB/SPF techniques have very complex geometries, exhibit highly non-linear material behavior, and are subject to large irreversible strains. Thus, there exists the possibility of many deformation-induced instabilities, such as necking, grooving, buckling and shear localization, which substantially weaken the structural integrity of the part.
The stringent requirements for both the external aerodynamic shape and internal structure of hollow airfoils present another problem in the manufacture of such parts. In order to produce the desired final shape and thickness, the in-process shape (i.e., the shape and size of a part prior to superplastic deformation) must be known. The determination of the in-process shape has proven to be a difficult task.